Larsen, S.E.

Mikkelsen, T.

Mortensen, N.G.

Troen, I.

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 19, 2017Two-years-worth of turbulence data recorded by a sonic-anemometer-based dataacquisition systemMortensen, Niels Gylling; Larsen, S.E.; Troen, I.; Mikkelsen, Torben KroghPublished in:Extended abstractsPublication date:1987Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Mortensen, N. G., Larsen, S. E., Troen, I., & Mikkelsen, T. (1987). Two-years-worth of turbulence data recordedby a sonic-anemometer-based data acquisition system. In Extended abstracts (pp. 393-396). AmericanMeteorological Society.SIXTH SYMPOSIUM METEOROLOGICAL OBSERVATIONS AND INSTRUMENTATION January 12-16, 1987 New Orleans, La. Sponsored By AMERICAN METEOROLOGICAL SOCIETY WORLD METEOROLOGICAL ORGANIZATION Copyright© 1986, American Meteorological Society. . This copyright notice applies to only the overall· collection of papers; authors retain their individual rights and should be contacted directly for permission to use their material separately. Contact ANS for permission pertaining to the overall collection. The manuscripts reproduced in this collection of extended abstracts are unrefereed papers presented at the Sixth Symposium on Meteorological Observations and Instrumentation; their appear-ance in this collection does not constitute formal publication. AMERICAN METEOROLOGICAL SOCIETY 45 Beacon St., B~~ton:· ;rviass. 02108 U.S.A. 21. l TWO-YEARS-WORTH OF TURBULENCE DATA RECORDED BY A SONIC-ANEMOMETER-BASED DATA ACQUISITION SYSTEM Niels G. Mortensen, S-ren E. Larsen, Ib Troen, and Torben Mikkelsen Ris- National Laboratory DK-4000 Roskilde, Denmark 1. INTRODUCTION It has become increasingly important that studies in wind ener15Y, diffusion, and air-sea interaction are supported by high quality, remote measurements of atmospheric turbulence where such data must reliably accumulate over long periods. As a result, much emphasis has been placed on the development of durable fast-response meteorologi-cal instrumentation during recent years. In this paper we describe a new, commercially available, omni-directional sonic anemometer/thermometer. Based on wind tunnel tests an extensive calibra-tion procedure for the sonic is reported. Finally, field data collected over a two-year period from a horizontally homogeneous site in the western part of Denmark are described as both instrument validation and as an illustration of the utility of the sonic for providing long-term high quality data. 2. THE SONIC ANEMOMETER/THERMOMETER The three-dimensional sonic system in question is a Kaijo Denki model DAT-300 with an omni-directional sensor (TR-61B); see Fig. 1. The sensor consists of three pairs of acoustic trans-ducers, each with a length of 80 mm and a dia-meter of 20 mm• The sound path for each pair of transducers is inclined 45° to the vertical and has a length of 20 cm. Fig. 1. The Kaijo Denki TR-blB probe head. Left and right in the figure correspond to north and south, respectively. 393 Calibration of the sonic was performed in the wind tunnel of the Danish·Maritime Institute. During the calibration controlled wind speeds were allowable over the range of 3 to 20 m/s and turbulence levels were artificially screened to low levels. 2.1 Sonic Internal Flow Distortion Because the sonic probe is designed to be stationary in the field with a varying wind vector, the distortion of the flow speed and di-rection (induced by the transducers and their supporting rods) was examined under controlled wind tunnel conditions. The sonic was systemati-cally rotated 3600 in the horizontal and through the range of +5° to -5° in the vertical. The main characteristics of the flow deformation for hori-zontal, laminar conditions are depicted in Fig. 2 (a, b, c). It appears that the speed-up factor (defined as the ratio of the sonic-measured wind speed to the true wind speed) is roughly 1.00 when averaged over azimuth, even though a sys-tematic variation of one-degree factors on the _order of 10% was observed. With reference also to Fig. 1, the supporting rods reduce the wind speed· considerably, whereas the flow speeds up when di-rected unobstructed towards the transducers. The wind vector was found to be tilted upwards an average of about 1° for all azimuth directions; and both the tilt and horizontal d.eflection curves depend strongly on the geometry of the sensor. The sonic response to an inclined flow (non-horizontal wind vector) was investigated for both different flow directions and flow speeds by tilting the probe head in the tunnel. An example of the tilting of the wind vector for three characteristic azimuth angles is shown in Fig. 3. The sonic-derived wind vector is distorted most when the flow is in the direction of the sound paths. The flow is also significantly tilted for flow past the supportin~ rods and be-tween sound paths. Considering the three-dimen-sional geometry of the probe head, the angular response is relatively complicated. The overall effect of the wind vector tilting originating in the sonic probe head was found to be a reduction of the vertical wind speed component on the order of 10%. Speed-up factor A 1.05 0.90 u.. ............................................................................................................................... ....i Wind-vector tilt [deg] B _, 3 Wind-vector deflection [deg] c 2 _, -2 ........................................................................................................................ .....:. ........ ..... 0 90 180 270 360 Azimuth [deg] Fig. 2. The sonic response to a laminar, horizon-tal flow of 10 ms-1 for different azimuth angles in a wind tunnel. The resolution in azimuth is 1° • a) Speed-up factor • ii'~onic/ii'true' b) Posi-tive tilt indicates upward tilt or the wind vector. c) Positive deflection indicates clock-wise deflection of the wind vector. 2.2 External Alignment The sonic anemometer provides the user with measurements of the wind-vector components Uf, Vf, Wf, obtained in a three-dimensional fixed Cartesian coordinate system, the x-axis of which conventionally points in the easterly, the y-axis in the northerly, and the z-axis in the vertical direction. The desired turbulence statistics, however, in terms of the covariance matrix of the fluctuating velocity components u', v', w', are commonly referred to a Cartesian coordinate system, the x-axis of which is aligned in the mean wind direction whereas the mean cross- and vertical wind components are zero. We shall ref er to this coordinate system as externally aligned. To calculate the externally aligned quan-tities from the measurements in the fixed co-ordinate system, we first rotate the frame of 394 Sonic tilt - tilt of probe head (deg] 1.0 0.5 0.0 -0.5 Sonic tilt (deg] -1.0 -5 0 5 Fig. 3. The tilting of the flow induced by the sonic probe as a function of flow inclination. "Sonic tilt" is the actual flow tilt measured by the sonic less the flow tilt measured by the sonic for horizontal flow. reference around the z-axis of the probe head an angle e = tan-l(vf/uf ), thereby aligning the x-axis with the mean wind direction in the xy-plane of the probe head. In this turned coordinate sy-stem the mean wind speed becomes uxy• whereas the corresponding cross-wind component Vxy equals zero. Subsequently, we rotate around the y-axis an angle, '' such that ' = tan-1 (wf/u ), there-by obtaining that also w in the rea ig~d system becomes zero. Following these rotations, the covariance matrix~ of the aligned wind components, u~ v: w~ relate to the covariance matrix ~ of the fixed-frame unaligned wind components, Uf, Vf, Wf • according to (1) Here, 11. = ! ft• where !, and A are unit matrices for right-handed coordinate rotation around the y- and z-axes, respectively. The alignment procedure, as well as the wind-tunnel-derived calibration curves (Fig. 2), are coded into the data acquisition system and provide real-time calculations of the turbulent energies (;;t'2', ;t2, ;t°2), shear stresses (u 1w1 , v'w'), and a sound-virtual temperature-based heat flux (w 1 T~). 3. FIELD RESULTS 3.1 Experimental Set-up The sonic-anemometer system was employed from February 1983 to February 1985 in a field experiment in western Jutland, Denmark. The sonic probe was mounted at a height of 5.5 m on a 24-meter meteorological mast. Further, the mast con-tained fast-responding cup-anemometers at heights of 3.0, 9.9 and 23.8 m, wind vanes at 9.9 and 23.8 m, and pt-500 temperature sensors at 2.0 and 22.9 m. The temperature sensors gave the absolute temperature as well as the temperature difference between the two levels. The mean relative hu-midity (at screen height) and short-wave incoming radiation were measured for prolonged periods. The mast was in successful operation for about two years. In this period the sonic anemometer system operated continuously for about 85% of the time, giving consecutive 20-min averages of mean wind speed and direction, wind vector tilt, tem-perature and variances and covariances of u', v', w' and T~ (sound-virtual temperature). The samp-lin~ rate was 1 Hz. The profile instrumentation had a corresponding efficiency of more·than 95%. The basic data set thus consists of more than 40,000 20-min values from each sensor. The mast was situated in an agricultural area, mainly with corn fields. The area is ex-tremely flat and devoid of buildings and vegeta-tion other than the crops. The results presented below were obtained in a 135-degree sector around SW where the surface is rather horizontal and homogeneous. 3.2 Results from the Sonic-Anemometer Figure 4 (a,b) shows the ratio of standard deviation of the vertical wind velocity fluctua-tions and the friction velocity as a function of surface layer stability in terms of z/L, where z is the height (5. 5 m) and L is the Monin-Obukhov length. The measurements are shown as 0 10-2 10-1 100 101 5 4 ; ; 3 ,, ; -----------/1 2 0 10-2 10-1 100 101 z/L Fib. 4. 0w/u* as function of z/L for unstable (a) and stable (b) conditions. The curves are from Merry & Panofsky (1976) (dashed) and Panofsky & Dutton (1984) (full line). 395 mean values of aw/u* in lo~arithmically equi-distant classes of z/L, and the standard devia-tion in each class is indicated by bars. The data are compared with relations suggested by Merry and Panofsky (1976) and Panofsky and Dutton (1984). In the unstable regime the measurements agree rather well with both relations though the data points are shifted slightly towards lower values of <Jw/U*• On the stable side (Fig. 4b) a~/u* is seen to be more or less constant to a z/L of about 1-2 (L ~ 3-5 m), as sui.i:;ested by Panofsky and Dutton (1984). For very stable con-ditions the scatter increases, but the data in-dicate an increasing ratio. The ratios au/u*• av/u* and a.,,/u* for near neutral stability were found to be 2.56, 1.83, and 1.15, respectively. These values are within 10% of the average values given in Panofsky and Dutton (1984, Table 7.1): 2.39, 1.92, and 1.25. 3.3 Results from Sonic and Profile Instrumen-tation Figure 5 compares the 20-min averaged wind speed at 5.5 m, measured by the sonic anemometer, with the corresponding wind speed derived from the profile instrumentation. The agreement be-tween sonic- and profile-derived wind speeds is seen to be excellent. Furthermore, the ratio of profile and sonic wind speeds has been analysed as a function of time, wind direction, wind speed and stability. These investigations showed that i) no drift in calibration of the sonic system could be discerned during 19 months of operation, ii) the correction of the sonic horizontal wind speeds (Fig. 2a) effectively removes any varia-tion with wind direction of the ratio ~uplUSonic• iii) the mean overspeeding correction of the cup-anemometers amounts to about 1% and, when Profile wind speed [m/s] 15 10 5 5 10 15 20 Sonic wind speed (m/s] Fib• 5. Profile-derived versus sonic wind speeds at 5.5 m a.g.1. Standard deviation of each 1 m/s-class indicated. applied, makes the ratio practically invariant with wind speed and stability. As an example of the application of the sonic anemometer system, we will finally present a climatological investigation of the flux-pro-f ile relationship for momentum. The dimensionless gradient of wind speed in the surface layer, ~. is by definition given as z i)u (2) where k is the von Karman constant, uw is the friction velocity and i)u/l)z is the gradient of mean wind speed at height z. l)u/l)z was found by differentiating mathematical expressions fitted to the wind-speed profile. The friction velocity is obtained direct!) from the sonic anemometer as ~{(<u'w'>)2+(<v'w'> 2. Prescribing a value of unity for <tM(O) at neutral conditions leads to a von Karman constant of 0.375, which is used in the following. Figure 6 shows the dimensionless wind speed gradient as a function of atmospheric stability in terms of the gradient Richardson number. Again, the number of observations allows us to present the data in logarithmically equidi-stant classes of stability. The standard devia-tion in each class is given by bars. The Richard-1.5 1.0 0.5 0.0 2 10- 101 -Ri Fig. 6. The dimensionless gradient of wind speed as a function of gradient Richardson number for unstable conditions. son number was chosen as scaling parameter be-cause it turned out to be a better statistic for describing ~ in the unstable ret;ime (Mortensen, 1985). Also shown is a Businger-'Dyer-like re-presentation of ~ for unstable conditions ~(Ri) = (1-a•Ri)n Ri < 0 (3) In our case the value of 12 for a and an exponent of -1/3 yields good agreement with the measure-ments. The result of a similar analysis in the stable regime is shown in Fig. 7. For Richardson numbers less than 0.15 the measurements are de-· scribed well by a relation of the form: 0 < Ri < 0 .15 (4) A value of 4.5 was used for b in Fig. 7. 396 cl>w(Ri) 1111 I Ri Fig. 7. As Fig. 6, but for stable conditions. 4. CONCLUSION The described sonic-anemometer and data acquisition system have proven a very sturdy and reliable turbulence instrument for unattended operation over prolonged periods. This is true for the above-mentioned field experiment and also for several air-sea interaction, diffusion and wind engineering studies, in which the system has been employed under a wide range of meteorologi-cal conditions. We presently believe that the i.reatest un-certainty in the turbulence statistics obtained with the system is associated with the vertical angular response of the sensor, i.e. the suppres-sion of the vertical wind velocity component. Further investigations will be conducted to re-solve this difficulty in more detail. The gene-rally good agreement between our observations and previously established relationships is en-couraging to this continued effort. 5. ACKNOWLEDGEMENTS The authors wish to thank Dr. G. Geernaert, NRL, for many valuable comments. 6. REFERENCES Merry, M. and H.A. Panofsky, 1976: Stati-stics of vertical motion over land and water. Quart. J. Roy. Meteor. Soc., 102, 255-260. Mortensen, N.G., 1985: Dimensionless gra-dients of wind speed and temperature in the at-mospheric surface layer. (In Danish). Ris '1)-I-209. [Available from Ris ~National Laboratory, DK-4000, Roskilde, Denmark). Panofsky, M.A. and J.A. Dutton, 1984: At-mospheric turbulence, John Wiley & Sons, New York. 

Two-years-worth of turbulence data recorded by a sonic-anemometer-based data acquisition system

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Two-years-worth of turbulence data recorded by a sonic-anemometer-based data acquisition system

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